This invention relates to radio frequency signalling systems. More particularly, the invention relates to simulcast radio frequency signalling systems and the problem of delay dispersion.
In paging and other radio communications systems, one primary transmitter often does not give sufficient coverage over a designated geographic area. In these instances, simulcast transmitters are used to fill in areas where coverage by the primary transmitter is insufficient or otherwise marginal. For example, FIG. 1A shows a conventional simulcast paging system which includes a primary transmitter 10 and a secondary transmitter 20. Transmitter 10 transmits information signals, either digital or analog, which include selective calling signals and message signals. These information signals are typically provided to secondary transmitter 20 by a telephone wire link 30 which connects primary transmitter 10 to secondary transmitter 20. In the conventional simulcast paging system of FIG. 1A, the information signals are transmitted simultaneously by the primary and secondary stations. Since there is a finite time delay associated with wire link 30 between the time at which the information signals from primary transmitter 10 enter link 30 and the time such information signals arrive at secondary transmitter 20, it is necessary to provide a time delay circuit 12 at the output of primary transmitter 10 such that the primary station antenna 14 and secondary station antenna 24 radiate the same information signals at the same time. For such simultaneous transmission to occur, the time delay exhibited by time delay circuit 12 is selected to be equal to the propagation delay of link 30.
FIG. 1B is a representation of a typical radiation pattern exhibited by the simulcast system of FIG. 1A. Primary transmitter 10 and secondary transmitter 20 are separated by a distance of 5 miles in this example. Primary transmitter 10 generates a radiation pattern 40 which covers primary coverage area 45. Secondary transmitter 20 generates a radiation pattern 50 which covers a secondary coverage area 55. Primary coverage area 45 and secondary area 55 overlap in overlap coverage area 60 in which signals from both primary transmitter 10 and secondary transmitter 20 are present. Depending on terrain and other propagation factors, the primary signal from the primary transmitter 10 will also be present with significant signal strengths in various portions of the secondary coverage area 55. This can cause interference problems for a receiver in the secondary coverage area 55 when such receiver moves from a first location 62 to a second location 64 which is shadowed from secondary transmitter 20.
It is seen that there are two possible signal paths over which information signals can reach a receiver located at second location 64. That is, under most conditions, the secondary signal would reach the receiver at location 64 over a path B which is the line between secondary transmitter 20 and location 64, a distance of approximately 2 miles in this example. However, under the shadow conditions described above, it is likely that information signals from the primary transmitter 10 will reach a receiver located at location 64 directly over a path A, which is the line between primary transmitter 10 and location 64, a distance of approximately 6 miles in this example.
Now continuing this example with more detail, assume that primary transmitter 10 is transmitting digital paging signals and that secondary transmitter 20 is simulcasting such digital paging signals. As per the system arrangement of FIG. 1A, both primary antenna 14 and secondary antenna 24 radiate information bits in sync with each other. Generally, receivers in the primary coverage area 45 are synchronized to primary transmitter 10 and receivers in the secondary coverage area 55 are synchronized to the secondary transmitter 20. If a receiver is located in the secondary coverage area 55 at an arbitrary first location 62 at which reception of the digital paging signals from the secondary transmitter 20 is relatively strong, then the receiver receives the secondary signal satisfactorily. If, however, the receiver is moved from such first location 62 to a second location 64 in the secondary coverage area 55 at which the secondary signal is shadowed, then the primary signal over path A may actually be stronger than the secondary signal at such second location 64 over path B. In this instance, the receiver will lose the secondary signal and, instead, receive the primary signal. If the particular digital paging system has a relatively low bit rate, for example a rate in the neighborhood of 600 bits per second (BPS), then it is likely that the receiver would not lose sync as it loses the secondary signal and acquires the primary signal. At such slow bit rates the propagation delay difference between the longer path A (6 miles) and the shorter path B (2 miles) is not a significant portion of a bit length. However, as the data rate is increased, for example to 10,000 through 20,000 BPS and above, then the propagation delay difference between the longer path A and the shorter path B is a substantial portion of a bit length. This effect is referred to as delay dispersion. (Delay dispersion relates to the difference in time delay between two or more components of a radio signal which arrive at a receiver via paths of unequal length.) Thus, with such higher data rates, when a receiver passes from first location 62 to a shadowed second location 64 in the secondary coverage area 55, it is possible that the receiver will experience delay dispersion and lose sync. When the receiver loses sync, valuable information signals are lost and cannot be recovered. Even if the receiver does not lose sync, under these delay dispersion conditions, the receiver may still experience "simulcast distortion" which causes bit errors in the reception of digital transmissions and distortion in terms of lessened intelligibility in analog or voice transmissions.
Contemporary simulcast systems suffer inter-symbol interference of higher baud rate signalling information when the radio frequency signal levels are close in amplitude (e.g., the receiver is in a "non-captured" state) and the time difference (differential delay) approaches one-half a symbol time period. This interference prohibits successful decoding of these higher baud rates. Many approaches have been tried to alleviate this problem, the most common one being to locate sites closer to each other. Regrettably, this solution does not offer an economical or reliable technical solution to the problem because as the differential delay increases, the difference in average signal levels required for capture and reliable decoding also increases. Thus, as more sites are added, the complexity of the non-capture signal overlapping and phasing requirements increase, thereby negatively impacting system performance and significantly increasing the overall system cost.